We study the structure, function, immunology and genetics of a large class of the related proteins, bacterial pili. Pili are rod-like assemblies of identical protein molecules (pilins). Some kinds of pili have tip-attached structures consisting of differentiated pilins (adhesions and other minor proteins). The principal function of pili is to attach bacteria to specific (usually carbohydrate) receptors on host cells. The pilus site for this adhesion is located on their distal tip. Essentially all pathogenic bacteria express pili. The tissue and organ tropisms of bacterial pathogens is largely determined by the binding specificity of their pili and the distribution of pilus receptors in the host. The objectives of our research are fundamental and applied. We seek to understand the structure-function relationships in and among pili on a molecular scale. To this end we have determined the amino acid sequences of a large number of pilus proteins using both chemical and genetic techniques. We have also determined the arrangement of protein subunits in pili using X-ray diffraction and crystallographic techniques. By aligning the amino acid sequences of a number of different pilins, adhesions and other pilus proteins we have discovered a large superfamily of related pili we call ``pilus superfamily I''. This family includes pili from a number of important pathogens, for example: Escherichia coli, Salmonella typhi, Klebsiella pneumoniae, Serratia marscescens, and Hemophilus influenzae. The alignment of these sequences has revealed conserved superfamily sequences involved in common peptide folding and protein assembly patterns, conserved adhesion family sequences invol ved in binding to pilus receptors, and variable sequences involved in inter-pilus binding and/or antigenic variation as an immune evasion mechanism.